Does the plugin have to be under that in order for me to edit?
Yes. The plugin has to be released under a GPL (or similar) licence. You should also attribute the original authors of both plugins and themes in the code or the accompanying readme.txt file.
Should I then just edit the plugin description and not change who authored the plugin?
That depends. You could try contacting the original plugin authors and asking them about this.
I’m trying to create an educational site. I really don’t want to mention wordpress and I found out that I don’t have to, but that the copyright has to remain intact. Is this the same with plugins that are released under a GPL license, cause I’ve noticed that almost all of them have a copyright.
I really don’t want to mention wordpress
Why not? It’s VERY easy for anyone with minimal knowledge to look at the code and see that it’s WP. Ditto plugins.
I have already answered this above…
How long about does it take for plugin creators to respond? Is it necessary to contact them, cause I have to get my site done very soon. I don’t really want to mention WordPress, because teachers (the group of people I’m making the website for) might become confused in some way.
How long about does it take for plugin creators to respond?
There’s no way to answer that fully. I would guess it takes as long as it needs to.
Is it necessary to contact them, cause I have to get my site done very soon.
If you are forking their plugins, yes – this is the ethical thing to do. Why do you want to “hide” evidence of WordPress being used? In reality, there is no way to do this fully.
Teachers probably won’t go through the source of a website, as far as I know. They’re not really comfortable with viewing source code. I will then just use plugins as they are and not edit them too much.
So if I’m going to attribute the original author(s), how would I do that? Should I mention the original plugin name?
I’d suggest something like:
Based on the original Foobar plugin by Joe Smith (http://example.com/)
in the plugin’s readme.txt file Description section.